Find a consistent solution for adding the robolab Debian package repository and its signing key

[lutzthies]

PRIORITY: LOW (possibly wontfix?)

Currently the entry is being set manually in /etc/apt/sources.list, however the gpg-key is being added during build script execution. One could add it to /etc/apt/trusted.gpg.d, but for the sake of flexibility in exchanging it, I'd like to keep the key on the server. Thus it makes sense to remove the entry of our Debian package repo from the sources.list and add it dynamically during the build as well in order to make the process consistent.